As this type of image sensor, there exists a CCD (Charge Coupled Device) type solid-state image sensor, for example. In recent years, in order to enable high-speed imaging, such a CCD type solid-state image sensor (hereinafter abbreviated as “CCD”) has been constructed capable of a short photographic cycle (e.g. 100 μs or less) such as high-speed imaging. At a moment of occurrence of a certain event showing a change in a photographic subject such as bursting of a balloon, collision of an iron ball or a stimulus to microorganisms, since the change in the event is rapid, it is possible to pick up images of the rapid change in the event in detail by using such high-speed imaging.
However, since the number of frames is limited, the photographic cycle is controlled to be short for a scene of interest on the basis of the occurrence of an event (see Patent Document 1, for example). It is desirable to pick up, before the occurrence of an event, images of a state before the change in the event. Then, images are picked up by controlling the photographic cycle to be long before the occurrence of the event, and the photographic cycle is controlled to be short after the occurrence of the event.
Incidentally, the driving of a CCD in high-speed imaging is divided broadly into a driving called “internal photography” and a driving called “external photography”. The driving of the CCD is started synchronously with an external signal taken in with the above occurrence of an event serving as external trigger, or an external signal generated by depression of an imaging button. In the above “internal photography”, control is performed to stop the driving of the CCD after picking up data for a predetermined number of frames (e.g. 100 frames) from a start of driving of the CCD. Conversely, in the above “external photography”, when an external signal is received after a start of driving of the CCD, control is performed to stop the driving of the CCD synchronously therewith.
In order to realize the high-speed imaging noted above, a conventional, ordinary image sensor of the CCD type or CMOS type cannot cope with a high-speed imaging having a photographing rate at 1.0×106 frames per second (1,000,000 frames per second), and therefore an image sensor of a special structure called “in-situ storage image sensor” is used (see Patent Document 2 and Nonpatent Document 1, for example).
The in-situ storage image sensor (IS-CCD: Image Storage-CCD), as shown in FIG. 2, includes, for each photodiode 11, storage CCDs 12 and vertical transfer CCDs 13 for predetermined frames (e.g. 100 frames). Signal charges generated from the photodiodes 11 during image pickup are stored while being successively transferred to the storage CCDs 12, and further to the vertical transfer CCDs 13. And images are acquired by successively transferring the signal charges for a predetermined number of frames stored in the storage CCDs 12 and vertical transfer CCDs 13 to a horizontal transfer CCD 15 for reading. Signal charges having exceeded the predetermined frames during the image pickup are discarded, and the latest signal charges for the predetermined frames are always stored in the storage CCDs 12 and vertical transfer CCDs 13. Therefore, when the transfer of the signal charges in the storage CCDs 12 and vertical transfer CCDs 13 is stopped at the end of the imaging, images corresponding to the predetermined number of the latest frames are obtained from the time going back by the predetermined frames in time from the end point of time.
[Patent Document 1]
Unexamined Patent Publication No. 2004-15612 (pages 1-16, FIGS. 1-10)
[Patent Document 2]
Unexamined Patent Publication No. 2001-345441
[Nonpatent Document 1]
Kondo and five others, “Development of “HyperVision HPV-1” High-speed Video Camera”, Shimadzu Review,
Shimadzu Review Editorial Department, published Sep. 30, 2005, Vol. 62, No. 1.2, p. 79-86